The Physics
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Frequency of the Cosmic Microwave Background

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Giancoli, Douglas. Physics 5th Revised Edition. 2002: 1072. "They made precise measurements at a wavelength λ = 7.35 cm, which is the microwave region of the electromagnetic spectrum." 408 GHz
Glossary. Cosmic Journeys. "Cosmic background radiation; primal glow The background of radiation mostly in the frequency range 3 × 108 to 3 × 1011 Hz … discovered in space in 1965. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself." 0.3–300 GHz
S.T. Staggs, N.C. Jarosik, S.S. Meyer, and D.T. Wilkinson. An Absolute Measurement Of The Cosmic Microwave Background Radiation Temperature At 10.7 GHz [pdf]. The Astrophysical Journal, 473: L1–L4, 1996 December 10. "The Far-Infrared Absolute Spectrophotometer (FIRAS) instrument on the COBE satellite measured the cmBR spectrum between 60 and 630 GHz very precisely (Fixsen et al. 1996), determining TCMBR to be 2.728 ± 0.004 K (95% CL) and constraining y < 1.5 × 10−5 and µ < 9 × 10−5." 60–630 GHz
Parker, Sybil P. McGraw Hill Encyclopedia of Astronomy. New York: McGraw Hill, 1983. "[graph]" 530 Hz
Scott, Douglas. Questions by e-mail. University of British Columbia. "For your information: there are about 412 cmB photons per cubic cm (with an uncertainty of about 1); the energy density is the equivalent of 0.261 electron Volts per cubic cm (again uncertain by about +/-1 in the last decimal place); the equivalent mass density is 4.66 × 10−31 kilogrammes per cubic metre; the peak of the spectrum is at a frequency of 160.4 GHz (uncertain by about +/-0.1); and the peak intensity of the background is about 385 MJy/Sr (that's MegaJanskys per Steradian, which is not a unit you meet everyday!)." 160.4 GHz

The Cosmic Microwave Background (CMB) is the isotropic, electromagnetic radiation which resulted from the explosion of the universe between 15 and 18 billion years ago. This theory, accepted by many but not all, is called The Big Bang theory. The Big Bang was the explosion of the universe from the extremely small, dense, and hot conditions of the early universe. The photons and baryons present in this environment formed plasma, a gas of ionized matter united with the radiation also found in the atmosphere. After the universe exploded, the temperature decreased and the universe began to expand very rapidly. Particles such as electrons, neutrinos, protons, and neutrons, which were non-existent in the era before the explosion due to the extremely high temperatures began to form in the cooler temperatures. For the next 300,000 years, the particles scattered around the universe. The universe was opaque because the photons constantly bumped into the additional particles, such as the electrons, protons, and neutrinos, thus not allowing the photons to travel far distances. Anytime the protons and electrons attempted to combine, the photons had enough energy to rip them apart. Eventually, the temperature decreased so greatly that it was possible for the electrons and protons to combine without intervention from the photons, and neutral hydrogen atoms were formed. This period in time is referred to as "recombination.""Recombination"turned the formerly opaque universe transparent. In turn, matter and radiation were disunited. The photons were able to travel throughout the universe, free from interaction with matter. The vast array of photons that roam the universe is now what we refer to as Cosmic Microwave Background.

Cosmic Microwave Background is the greatest evidence in support of the Big Bang theory. Due to the constant expansion of the universe, the radiation had cooled to 2.73 K. The cmB is isotropic, meaning that it is uniform throughout the universe in all directions. The cmB spectrum is a perfect blackbody, or an object capable of absorbing all the electromagnetic radiation that it comes in contact with. By observing the cmB, scientists can learn a great deal about the universe after the big bang. The cmB does not have one specific frequency, but many frequencies throughout the microwave range. The peak frequency is 160.4 GHz. The frequencies range from 0.3 GHz to 630 GHz.

Heather Friedberg -- 2004